How would you tell a friend how to get to your house for the first time?

You might start by giving your friend the name of your street. "I live on Pine Street," you might say. But that information alone wouldn't be much help. Because your friend could travel from one end of Pine Street to the other and still not know where to find your house.

So what you'd probably do next is tell your friend where on Pine Street you live. You might say, "I live at the intersection of Pine Street and 16th Avenue," or "I live at 1615 Pine Street." What you have done by giving your address or intersection is give your friend coordinates that can be used to find your house. One of the coordinates, let's say Pine Street, will tell your friend how far north or south to travel. And the other coordinate, let's say 16th Avenue (or the address 1615) will tell your friend how far east or west to travel.

It sounds pretty simple when we think about using street addresses or intersections as coordinates to help a friend find your house. But, in fact, the use of coordinates is exactly what sailors at sea have done for many centuries to learn where they were and where they needed to go. And let's face it: it's a lot more confusing to figure out where you are in the middle of the ocean than it is on a city street.

Many hundreds of years ago, sailors and geographers created two types of measurements to serve as coordinates anywhere in the world:

Latitude gives the location of a place north or south of the equator. The equator is the 0º latitude line. The North and South Poles are each at 90º latitude lines (north and south of the equator).

Longitude gives the location of a place east or west of a north-south line called the prime meridian (at the Greenwich, England observatory). Longitude lines go from 0º at the prime meridian to 180º at the other side of the world at the International Date Line.

Look at a globe or world map. You will see the latitude and longitude lines that neatly divide the world into little squares. Find your hometown and see if you can figure out its latitude and longitude. With this information, you can tell anyone exactly where in the world you are.

That sounds pretty simple, right? But what if you're on a ship at sea, don't know where you are, and need to get to a certain latitude and longitude? How would you find your way?

You might say that the sun and stars could help you find your way. And you'd be at least partly right. You see, the noon-day sun is higher in the sky at the equator than it is farther north or south. So, measuring the angle between the sun and the horizon can help determine latitude. The astrolabe, cross-staff, back-staff, and sextant were all devices developed centuries ago to help sailors make these measurements and find their latitude.

But the position of the sun wouldn't help with longitude. And while the position of stars, planets, or moons might help, sailors didn't have accurate enough measuring devices to track them. So sailors turned to another measurement: time. Because the earth makes a full turn (360º) every 24 hours - or 15º each hour - sailors thought they could tell how far east or west they were by figuring out the difference between the time at their home port and the time on the ship.

That sounds simple, right? But unfortunately, there just wasn't a clock accurate enough to keep good time on a rocking ship out in the ocean facing changes in air pressure, temperature, moisture, and latitude. That is, there wasn't until the 1700s, when clockmaker John Harrison created the chronometer, a small clock that was so accurate it could show sailors exactly where in the world they were.

Today, we still use latitude and longitude to find our way in the world. And, you may be surprised to learn, we still use time as well. It's gotten a little more sophisticated of course - the Global Positioning System uses 24 satellites with atomic clocks that can determine where anyone is as well as where they are going.

Assignments:

• Learn more about one of the tools used to find latitude. Write a 2-page paper about what you learn.

• Lear more about the quest to determine longitude using the time. Write a 2-page paper about what you learn.

• Learn more about the Global Positioning System and how it is used. Write a 2- to 3-page paper about what you learn.

Teachers: For more information:

• NOVA's "Lost at Sea - The Search for Longitude," can be found at http://www.pbs.org/wgbh/nova/longitude/

• Hammond Map has an online information section for students at www.hammondmap.com/latlong.html

Check out past Trade Topics entries!